diff --git a/blocks/block_test.go b/blocks/block_test.go
index 26fe5eba1..972ae8547 100644
--- a/blocks/block_test.go
+++ b/blocks/block_test.go
@@ -39,13 +39,11 @@ func TestBlock(t *testing.T) {
 		BaseTx:   randBaseTx(),
 		Fee:      RandUInt64Exp(),
 		UnbondTo: RandUInt64Exp(),
-		Amount:   RandUInt64Exp(),
 	}
 
 	unbondTx := &UnbondTx{
 		BaseTx: randBaseTx(),
 		Fee:    RandUInt64Exp(),
-		Amount: RandUInt64Exp(),
 	}
 
 	timeoutTx := &TimeoutTx{
diff --git a/blocks/tx.go b/blocks/tx.go
index e43f3b8b9..e7d248e95 100644
--- a/blocks/tx.go
+++ b/blocks/tx.go
@@ -20,7 +20,7 @@ Validation Txs:
 
 type Tx interface {
 	Signable
-	GetSequence() uint64
+	GetSequence() uint
 }
 
 const (
@@ -83,7 +83,7 @@ func ReadTx(r io.Reader, n *int64, err *error) Tx {
 //-----------------------------------------------------------------------------
 
 type BaseTx struct {
-	Sequence uint64
+	Sequence uint
 	Signature
 }
 
@@ -100,7 +100,7 @@ func (tx BaseTx) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
-func (tx *BaseTx) GetSequence() uint64 {
+func (tx *BaseTx) GetSequence() uint {
 	return tx.Sequence
 }
 
diff --git a/common/random.go b/common/random.go
index 2588db623..3b46ef67e 100644
--- a/common/random.go
+++ b/common/random.go
@@ -58,6 +58,10 @@ func RandUInt64() uint64 {
 	return uint64(rand.Uint32())<<32 + uint64(rand.Uint32())
 }
 
+func RandUInt() uint {
+	return uint(rand.Int())
+}
+
 // Distributed pseudo-exponentially to test for various cases
 func RandUInt16Exp() uint16 {
 	bits := rand.Uint32() % 16
diff --git a/consensus/consensus.go b/consensus/consensus.go
index 4e616ad5c..de7db1440 100644
--- a/consensus/consensus.go
+++ b/consensus/consensus.go
@@ -229,8 +229,8 @@ func (conR *ConsensusReactor) Receive(chId byte, peer *p2p.Peer, msgBytes []byte
 			if vote.Height != rs.Height || vote.Height != ps.Height {
 				return
 			}
-			index, ok := rs.Validators.GetIndexById(vote.SignerId)
-			if !ok {
+			index, val := rs.Validators.GetById(vote.SignerId)
+			if val == nil {
 				log.Warning("Peer gave us an invalid vote.")
 				return
 			}
@@ -348,8 +348,8 @@ OUTER_LOOP:
 		if prs.Step <= RoundStepVote {
 			index, ok := rs.Votes.BitArray().Sub(prs.Votes).PickRandom()
 			if ok {
-				valId, ok := rs.Validators.GetIdByIndex(uint32(index))
-				if ok {
+				valId, val := rs.Validators.GetByIndex(uint32(index))
+				if val != nil {
 					vote := rs.Votes.GetVote(valId)
 					msg := p2p.TypedMessage{msgTypeVote, vote}
 					peer.Send(VoteCh, msg)
@@ -365,8 +365,8 @@ OUTER_LOOP:
 		if prs.Step <= RoundStepPrecommit {
 			index, ok := rs.Precommits.BitArray().Sub(prs.Precommits).PickRandom()
 			if ok {
-				valId, ok := rs.Validators.GetIdByIndex(uint32(index))
-				if ok {
+				valId, val := rs.Validators.GetByIndex(uint32(index))
+				if val != nil {
 					vote := rs.Precommits.GetVote(valId)
 					msg := p2p.TypedMessage{msgTypeVote, vote}
 					peer.Send(VoteCh, msg)
@@ -381,8 +381,8 @@ OUTER_LOOP:
 		// If there are any commits to send...
 		index, ok := rs.Commits.BitArray().Sub(prs.Commits).PickRandom()
 		if ok {
-			valId, ok := rs.Validators.GetIdByIndex(uint32(index))
-			if ok {
+			valId, val := rs.Validators.GetByIndex(uint32(index))
+			if val != nil {
 				vote := rs.Commits.GetVote(valId)
 				msg := p2p.TypedMessage{msgTypeVote, vote}
 				peer.Send(VoteCh, msg)
diff --git a/consensus/pol.go b/consensus/pol.go
index 1308b3744..d1eb888e8 100644
--- a/consensus/pol.go
+++ b/consensus/pol.go
@@ -55,17 +55,17 @@ func (pol *POL) Verify(vset *ValidatorSet) error {
 		if _, seen := seenValidators[sig.SignerId]; seen {
 			return Errorf("Duplicate validator for vote %v for POL %v", sig, pol)
 		}
-		validator := vset.GetById(sig.SignerId)
-		if validator == nil {
+		_, val := vset.GetById(sig.SignerId)
+		if val == nil {
 			return Errorf("Invalid validator for vote %v for POL %v", sig, pol)
 		}
-		if !validator.VerifyBytes(voteDoc, sig) {
+		if !val.VerifyBytes(voteDoc, sig) {
 			return Errorf("Invalid signature for vote %v for POL %v", sig, pol)
 		}
 
 		// Tally
-		seenValidators[validator.Id] = struct{}{}
-		talliedVotingPower += validator.VotingPower
+		seenValidators[val.Id] = struct{}{}
+		talliedVotingPower += val.VotingPower
 	}
 
 	for i, sig := range pol.Commits {
@@ -75,20 +75,20 @@ func (pol *POL) Verify(vset *ValidatorSet) error {
 		if _, seen := seenValidators[sig.SignerId]; seen {
 			return Errorf("Duplicate validator for commit %v for POL %v", sig, pol)
 		}
-		validator := vset.GetById(sig.SignerId)
-		if validator == nil {
+		_, val := vset.GetById(sig.SignerId)
+		if val == nil {
 			return Errorf("Invalid validator for commit %v for POL %v", sig, pol)
 		}
 
 		commitDoc := BinaryBytes(&Vote{Height: pol.Height, Round: round,
 			Type: VoteTypeCommit, BlockHash: pol.BlockHash}) // TODO cache
-		if !validator.VerifyBytes(commitDoc, sig) {
+		if !val.VerifyBytes(commitDoc, sig) {
 			return Errorf("Invalid signature for commit %v for POL %v", sig, pol)
 		}
 
 		// Tally
-		seenValidators[validator.Id] = struct{}{}
-		talliedVotingPower += validator.VotingPower
+		seenValidators[val.Id] = struct{}{}
+		talliedVotingPower += val.VotingPower
 	}
 
 	if talliedVotingPower > vset.TotalVotingPower()*2/3 {
diff --git a/consensus/state.go b/consensus/state.go
index 74b25218b..f79548e4c 100644
--- a/consensus/state.go
+++ b/consensus/state.go
@@ -92,7 +92,7 @@ func (cs *ConsensusState) updateToState(state *State) {
 	cs.Step = RoundStepStart
 	cs.StartTime = state.CommitTime.Add(newBlockWaitDuration)
 	cs.Validators = validators
-	cs.Proposer = validators.GetProposer()
+	cs.Proposer = validators.Proposer()
 	cs.Proposal = nil
 	cs.ProposalBlock = nil
 	cs.ProposalBlockPartSet = nil
@@ -135,7 +135,7 @@ func (cs *ConsensusState) setupRound(round uint16) {
 	cs.Round = round
 	cs.Step = RoundStepStart
 	cs.Validators = validators
-	cs.Proposer = validators.GetProposer()
+	cs.Proposer = validators.Proposer()
 	cs.Proposal = nil
 	cs.ProposalBlock = nil
 	cs.ProposalBlockPartSet = nil
diff --git a/consensus/vote_set.go b/consensus/vote_set.go
index 851b15ec8..8dd2b8855 100644
--- a/consensus/vote_set.go
+++ b/consensus/vote_set.go
@@ -63,7 +63,7 @@ func (vs *VoteSet) AddVote(vote *Vote) (bool, error) {
 	}
 
 	// Ensure that signer is a validator.
-	val := vs.vset.GetById(vote.SignerId)
+	_, val := vs.vset.GetById(vote.SignerId)
 	if val == nil {
 		return false, ErrVoteInvalidAccount
 	}
@@ -89,12 +89,11 @@ func (vs *VoteSet) addVote(vote *Vote) (bool, error) {
 
 	// Add vote.
 	vs.votes[vote.SignerId] = vote
-	voterIndex, ok := vs.vset.GetIndexById(vote.SignerId)
-	if !ok {
+	voterIndex, val := vs.vset.GetById(vote.SignerId)
+	if val == nil {
 		return false, ErrVoteInvalidAccount
 	}
 	vs.votesBitArray.SetIndex(uint(voterIndex), true)
-	val := vs.vset.GetById(vote.SignerId)
 	totalBlockHashVotes := vs.votesByBlockHash[string(vote.BlockHash)] + val.VotingPower
 	vs.votesByBlockHash[string(vote.BlockHash)] = totalBlockHashVotes
 	vs.totalVotes += val.VotingPower
diff --git a/merkle/iavl_tree.go b/merkle/iavl_tree.go
index 514170259..0c743542b 100644
--- a/merkle/iavl_tree.go
+++ b/merkle/iavl_tree.go
@@ -39,7 +39,7 @@ func NewIAVLTree(keyCodec, valueCodec Codec, cacheSize int, db DB) *IAVLTree {
 
 // The returned tree and the original tree are goroutine independent.
 // That is, they can each run in their own goroutine.
-func (t *IAVLTree) Copy() *IAVLTree {
+func (t *IAVLTree) Copy() Tree {
 	if t.ndb != nil && !t.root.persisted {
 		panic("It is unsafe to Copy() an unpersisted tree.")
 		// Saving a tree finalizes all the nodes.
diff --git a/merkle/types.go b/merkle/types.go
index a3fa9835a..bba0eee11 100644
--- a/merkle/types.go
+++ b/merkle/types.go
@@ -11,8 +11,8 @@ type Tree interface {
 	HashWithCount() (hash []byte, count uint64)
 	Hash() (hash []byte)
 	Save() (hash []byte)
-	Checkpoint() (checkpoint interface{})
-	Restore(checkpoint interface{})
+	Load(hash []byte)
+	Copy() Tree
 	Iterate(func(key interface{}, value interface{}) (stop bool)) (stopped bool)
 }
 
diff --git a/state/account.go b/state/account.go
index 91405209f..f1e4fd89b 100644
--- a/state/account.go
+++ b/state/account.go
@@ -57,7 +57,7 @@ func (account Account) Verify(o Signable) bool {
 
 type AccountDetail struct {
 	Account
-	Sequence uint64
+	Sequence uint
 	Balance  uint64
 	Status   byte
 }
@@ -65,7 +65,7 @@ type AccountDetail struct {
 func ReadAccountDetail(r io.Reader, n *int64, err *error) *AccountDetail {
 	return &AccountDetail{
 		Account:  ReadAccount(r, n, err),
-		Sequence: ReadUInt64(r, n, err),
+		Sequence: ReadUVarInt(r, n, err),
 		Balance:  ReadUInt64(r, n, err),
 		Status:   ReadByte(r, n, err),
 	}
@@ -73,12 +73,30 @@ func ReadAccountDetail(r io.Reader, n *int64, err *error) *AccountDetail {
 
 func (accDet AccountDetail) WriteTo(w io.Writer) (n int64, err error) {
 	WriteBinary(w, accDet.Account, &n, &err)
-	WriteUInt64(w, accDet.Sequence, &n, &err)
+	WriteUVarInt(w, accDet.Sequence, &n, &err)
 	WriteUInt64(w, accDet.Balance, &n, &err)
 	WriteByte(w, accDet.Status, &n, &err)
 	return
 }
 
+//-------------------------------------
+
+var AccountDetailCodec = accountDetailCodec{}
+
+type accountDetailCodec struct{}
+
+func (abc accountDetailCodec) Encode(accDet interface{}, w io.Writer, n *int64, err *error) {
+	WriteBinary(w, accDet.(*AccountDetail), n, err)
+}
+
+func (abc accountDetailCodec) Decode(r io.Reader, n *int64, err *error) interface{} {
+	return ReadAccountDetail(r, n, err)
+}
+
+func (abc accountDetailCodec) Compare(o1 interface{}, o2 interface{}) int {
+	panic("AccountDetailCodec.Compare not implemented")
+}
+
 //-----------------------------------------------------------------------------
 
 type PrivAccount struct {
diff --git a/state/account_test.go b/state/account_test.go
index a9af1761e..20ce6021f 100644
--- a/state/account_test.go
+++ b/state/account_test.go
@@ -15,14 +15,14 @@ func TestSignAndValidate(t *testing.T) {
 	t.Logf("msg: %X, sig: %X", msg, sig)
 
 	// Test the signature
-	if !account.Verify(msg, sig) {
+	if !account.VerifyBytes(msg, sig) {
 		t.Errorf("Account message signature verification failed")
 	}
 
 	// Mutate the signature, just one bit.
 	sig.Bytes[0] ^= byte(0x01)
 
-	if account.Verify(msg, sig) {
+	if account.VerifyBytes(msg, sig) {
 		t.Errorf("Account message signature verification should have failed but passed instead")
 	}
 }
diff --git a/state/state.go b/state/state.go
index b5d26d613..d7568797f 100644
--- a/state/state.go
+++ b/state/state.go
@@ -20,23 +20,11 @@ var (
 	ErrStateInvalidAccountStateHash    = errors.New("Error State invalid AccountStateHash")
 	ErrStateInsufficientFunds          = errors.New("Error State insufficient funds")
 
-	stateKey      = []byte("stateKey")
-	minBondAmount = uint64(1) // TODO adjust
+	stateKey                           = []byte("stateKey")
+	minBondAmount                      = uint64(1) // TODO adjust
+	defaultAccountDetailsCacheCapacity = 1000      // TODO adjust
 )
 
-type accountDetailCodec struct{}
-
-func (abc accountDetailCodec) Write(accDet interface{}) (accDetBytes []byte, err error) {
-	w := new(bytes.Buffer)
-	_, err = accDet.(*AccountDetail).WriteTo(w)
-	return w.Bytes(), err
-}
-
-func (abc accountDetailCodec) Read(accDetBytes []byte) (interface{}, error) {
-	n, err, r := new(int64), new(error), bytes.NewBuffer(accDetBytes)
-	return ReadAccountDetail(r, n, err), *err
-}
-
 //-----------------------------------------------------------------------------
 
 // NOTE: not goroutine-safe.
@@ -45,25 +33,31 @@ type State struct {
 	Height         uint32 // Last known block height
 	BlockHash      []byte // Last known block hash
 	CommitTime     time.Time
-	AccountDetails *merkle.TypedTree
+	AccountDetails merkle.Tree
 	Validators     *ValidatorSet
 }
 
 func GenesisState(db DB, genesisTime time.Time, accDets []*AccountDetail) *State {
 
 	// TODO: Use "uint64Codec" instead of BasicCodec
-	accountDetails := merkle.NewTypedTree(merkle.NewIAVLTree(db), BasicCodec, accountDetailCodec{})
-	validators := map[uint64]*Validator{}
+	accountDetails := merkle.NewIAVLTree(BasicCodec, AccountDetailCodec, defaultAccountDetailsCacheCapacity, db)
+	validators := []*Validator{}
 
 	for _, accDet := range accDets {
 		accountDetails.Set(accDet.Id, accDet)
-		validators[accDet.Id] = &Validator{
-			Account:     accDet.Account,
-			BondHeight:  0,
-			VotingPower: accDet.Balance,
-			Accum:       0,
+		if accDet.Status == AccountDetailStatusBonded {
+			validators = append(validators, &Validator{
+				Account:     accDet.Account,
+				BondHeight:  0,
+				VotingPower: accDet.Balance,
+				Accum:       0,
+			})
 		}
 	}
+
+	if len(validators) == 0 {
+		panic("Must have some validators")
+	}
 	validatorSet := NewValidatorSet(validators)
 
 	return &State{
@@ -89,16 +83,13 @@ func LoadState(db DB) *State {
 		s.CommitTime = ReadTime(reader, &n, &err)
 		s.BlockHash = ReadByteSlice(reader, &n, &err)
 		accountDetailsHash := ReadByteSlice(reader, &n, &err)
-		s.AccountDetails = merkle.NewTypedTree(merkle.LoadIAVLTreeFromHash(db, accountDetailsHash), BasicCodec, accountDetailCodec{})
-		var validators = map[uint64]*Validator{}
-		for reader.Len() > 0 {
-			validator := ReadValidator(reader, &n, &err)
-			validators[validator.Id] = validator
-		}
-		s.Validators = NewValidatorSet(validators)
+		s.AccountDetails = merkle.NewIAVLTree(BasicCodec, AccountDetailCodec, defaultAccountDetailsCacheCapacity, db)
+		s.AccountDetails.Load(accountDetailsHash)
+		s.Validators = ReadValidatorSet(reader, &n, &err)
 		if err != nil {
 			panic(err)
 		}
+		// TODO: ensure that buf is completely read.
 	}
 	return s
 }
@@ -108,17 +99,15 @@ func LoadState(db DB) *State {
 // is saved here.
 func (s *State) Save(commitTime time.Time) {
 	s.CommitTime = commitTime
-	s.AccountDetails.Tree.Save()
+	s.AccountDetails.Save()
 	var buf bytes.Buffer
 	var n int64
 	var err error
 	WriteUInt32(&buf, s.Height, &n, &err)
 	WriteTime(&buf, commitTime, &n, &err)
 	WriteByteSlice(&buf, s.BlockHash, &n, &err)
-	WriteByteSlice(&buf, s.AccountDetails.Tree.Hash(), &n, &err)
-	for _, validator := range s.Validators.Map() {
-		WriteBinary(&buf, validator, &n, &err)
-	}
+	WriteByteSlice(&buf, s.AccountDetails.Hash(), &n, &err)
+	WriteBinary(&buf, s.Validators, &n, &err)
 	if err != nil {
 		panic(err)
 	}
@@ -225,7 +214,7 @@ func (s *State) AppendBlock(b *Block) error {
 	if !bytes.Equal(s.Validators.Hash(), b.ValidationStateHash) {
 		return ErrStateInvalidValidationStateHash
 	}
-	if !bytes.Equal(s.AccountDetails.Tree.Hash(), b.AccountStateHash) {
+	if !bytes.Equal(s.AccountDetails.Hash(), b.AccountStateHash) {
 		return ErrStateInvalidAccountStateHash
 	}
 
@@ -235,7 +224,7 @@ func (s *State) AppendBlock(b *Block) error {
 }
 
 func (s *State) GetAccountDetail(accountId uint64) *AccountDetail {
-	accDet := s.AccountDetails.Get(accountId)
+	_, accDet := s.AccountDetails.Get(accountId)
 	if accDet == nil {
 		return nil
 	}
diff --git a/state/state_test.go b/state/state_test.go
index d45d01396..ed20efcc4 100644
--- a/state/state_test.go
+++ b/state/state_test.go
@@ -11,29 +11,30 @@ import (
 	"time"
 )
 
-func randAccountBalance(id uint64, status byte) *AccountBalance {
-	return &AccountBalance{
+func randAccountDetail(id uint64, status byte) *AccountDetail {
+	return &AccountDetail{
 		Account: Account{
 			Id:     id,
 			PubKey: CRandBytes(32),
 		},
-		Balance: RandUInt64(),
-		Status:  status,
+		Sequence: RandUInt(),
+		Balance:  RandUInt64(),
+		Status:   status,
 	}
 }
 
 // The first numValidators accounts are validators.
 func randGenesisState(numAccounts int, numValidators int) *State {
 	db := NewMemDB()
-	accountBalances := make([]*AccountBalance, numAccounts)
+	accountDetails := make([]*AccountDetail, numAccounts)
 	for i := 0; i < numAccounts; i++ {
 		if i < numValidators {
-			accountBalances[i] = randAccountBalance(uint64(i), AccountBalanceStatusNominal)
+			accountDetails[i] = randAccountDetail(uint64(i), AccountDetailStatusNominal)
 		} else {
-			accountBalances[i] = randAccountBalance(uint64(i), AccountBalanceStatusBonded)
+			accountDetails[i] = randAccountDetail(uint64(i), AccountDetailStatusBonded)
 		}
 	}
-	s0 := GenesisState(db, time.Now(), accountBalances)
+	s0 := GenesisState(db, time.Now(), accountDetails)
 	return s0
 }
 
@@ -42,10 +43,11 @@ func TestGenesisSaveLoad(t *testing.T) {
 	// Generate a state, save & load it.
 	s0 := randGenesisState(10, 5)
 	// Figure out what the next state hashes should be.
+	s0.Validators.Hash()
 	s0ValsCopy := s0.Validators.Copy()
 	s0ValsCopy.IncrementAccum()
 	nextValidationStateHash := s0ValsCopy.Hash()
-	nextAccountStateHash := s0.AccountBalances.Tree.Hash()
+	nextAccountStateHash := s0.AccountDetails.Hash()
 	// Mutate the state to append one empty block.
 	block := &Block{
 		Header: Header{
@@ -97,7 +99,7 @@ func TestGenesisSaveLoad(t *testing.T) {
 	if s0.Validators.TotalVotingPower() != s1.Validators.TotalVotingPower() {
 		t.Error("Validators TotalVotingPower mismatch")
 	}
-	if !bytes.Equal(s0.AccountBalances.Tree.Hash(), s1.AccountBalances.Tree.Hash()) {
-		t.Error("AccountBalance mismatch")
+	if !bytes.Equal(s0.AccountDetails.Hash(), s1.AccountDetails.Hash()) {
+		t.Error("AccountDetail mismatch")
 	}
 }
diff --git a/state/validator.go b/state/validator.go
index 0995b75c1..f57b3aaf6 100644
--- a/state/validator.go
+++ b/state/validator.go
@@ -4,8 +4,6 @@ import (
 	"io"
 
 	. "github.com/tendermint/tendermint/binary"
-	. "github.com/tendermint/tendermint/common"
-	"github.com/tendermint/tendermint/merkle"
 )
 
 // Holds state for a Validator at a given height+round.
@@ -47,126 +45,40 @@ func (v *Validator) WriteTo(w io.Writer) (n int64, err error) {
 	return
 }
 
-//-----------------------------------------------------------------------------
-
-// Not goroutine-safe.
-type ValidatorSet struct {
-	validators       map[uint64]*Validator
-	indexToId        map[uint32]uint64 // bitarray index to validator id
-	idToIndex        map[uint64]uint32 // validator id to bitarray index
-	totalVotingPower uint64
-}
-
-func NewValidatorSet(validators map[uint64]*Validator) *ValidatorSet {
-	if validators == nil {
-		validators = make(map[uint64]*Validator)
-	}
-	ids := []uint64{}
-	indexToId := map[uint32]uint64{}
-	idToIndex := map[uint64]uint32{}
-	totalVotingPower := uint64(0)
-	for id, val := range validators {
-		ids = append(ids, id)
-		totalVotingPower += val.VotingPower
+// Returns the one with higher Accum.
+func (v *Validator) CompareAccum(other *Validator) *Validator {
+	if v == nil {
+		return other
 	}
-	UInt64Slice(ids).Sort()
-	for i, id := range ids {
-		indexToId[uint32(i)] = id
-		idToIndex[id] = uint32(i)
-	}
-	return &ValidatorSet{
-		validators:       validators,
-		indexToId:        indexToId,
-		idToIndex:        idToIndex,
-		totalVotingPower: totalVotingPower,
-	}
-}
-
-func (vset *ValidatorSet) IncrementAccum() {
-	totalDelta := int64(0)
-	for _, validator := range vset.validators {
-		validator.Accum += int64(validator.VotingPower)
-		totalDelta += int64(validator.VotingPower)
-	}
-	proposer := vset.GetProposer()
-	proposer.Accum -= totalDelta
-	// NOTE: sum(v) here should be zero.
-	if true {
-		totalAccum := int64(0)
-		for _, validator := range vset.validators {
-			totalAccum += validator.Accum
-		}
-		if totalAccum != 0 {
-			Panicf("Total Accum of validators did not equal 0. Got: ", totalAccum)
+	if v.Accum > other.Accum {
+		return v
+	} else if v.Accum < other.Accum {
+		return other
+	} else {
+		if v.Id < other.Id {
+			return v
+		} else if v.Id > other.Id {
+			return other
+		} else {
+			panic("Cannot compare identical validators")
 		}
 	}
 }
 
-func (vset *ValidatorSet) Copy() *ValidatorSet {
-	validators := map[uint64]*Validator{}
-	for id, val := range vset.validators {
-		validators[id] = val.Copy()
-	}
-	return &ValidatorSet{
-		validators:       validators,
-		indexToId:        vset.indexToId,
-		idToIndex:        vset.idToIndex,
-		totalVotingPower: vset.totalVotingPower,
-	}
-}
-
-func (vset *ValidatorSet) GetById(id uint64) *Validator {
-	return vset.validators[id]
-}
+//-------------------------------------
 
-func (vset *ValidatorSet) GetIndexById(id uint64) (uint32, bool) {
-	index, ok := vset.idToIndex[id]
-	return index, ok
-}
+var ValidatorCodec = validatorCodec{}
 
-func (vset *ValidatorSet) GetIdByIndex(index uint32) (uint64, bool) {
-	id, ok := vset.indexToId[index]
-	return id, ok
-}
+type validatorCodec struct{}
 
-func (vset *ValidatorSet) Map() map[uint64]*Validator {
-	return vset.validators
+func (vc validatorCodec) Encode(o interface{}, w io.Writer, n *int64, err *error) {
+	WriteBinary(w, o.(*Validator), n, err)
 }
 
-func (vset *ValidatorSet) Size() uint {
-	return uint(len(vset.validators))
+func (vc validatorCodec) Decode(r io.Reader, n *int64, err *error) interface{} {
+	return ReadValidator(r, n, err)
 }
 
-func (vset *ValidatorSet) TotalVotingPower() uint64 {
-	return vset.totalVotingPower
-}
-
-// TODO: cache proposer. invalidate upon increment.
-func (vset *ValidatorSet) GetProposer() (proposer *Validator) {
-	highestAccum := int64(0)
-	for _, validator := range vset.validators {
-		if validator.Accum > highestAccum {
-			highestAccum = validator.Accum
-			proposer = validator
-		} else if validator.Accum == highestAccum {
-			if validator.Id < proposer.Id { // Seniority
-				proposer = validator
-			}
-		}
-	}
-	return
-}
-
-// Should uniquely determine the state of the ValidatorSet.
-func (vset *ValidatorSet) Hash() []byte {
-	ids := []uint64{}
-	for id, _ := range vset.validators {
-		ids = append(ids, id)
-	}
-	UInt64Slice(ids).Sort()
-	sortedValidators := make([]Binary, len(ids))
-	for i, id := range ids {
-		sortedValidators[i] = vset.validators[id]
-	}
-	return merkle.HashFromBinaries(sortedValidators)
+func (vc validatorCodec) Compare(o1 interface{}, o2 interface{}) int {
+	panic("ValidatorCodec.Compare not implemented")
 }
diff --git a/state/validator_set.go b/state/validator_set.go
new file mode 100644
index 000000000..38b9a4e0e
--- /dev/null
+++ b/state/validator_set.go
@@ -0,0 +1,117 @@
+package state
+
+import (
+	"io"
+
+	. "github.com/tendermint/tendermint/binary"
+	"github.com/tendermint/tendermint/merkle"
+)
+
+// Not goroutine-safe.
+type ValidatorSet struct {
+	validators       merkle.Tree
+	proposer         *Validator // Whoever has the highest Accum.
+	totalVotingPower uint64
+}
+
+func NewValidatorSet(vals []*Validator) *ValidatorSet {
+	validators := merkle.NewIAVLTree(BasicCodec, ValidatorCodec, 0, nil) // In memory
+	var proposer *Validator
+	totalVotingPower := uint64(0)
+	for _, val := range vals {
+		validators.Set(val.Id, val)
+		proposer = proposer.CompareAccum(val)
+		totalVotingPower += val.VotingPower
+	}
+	return &ValidatorSet{
+		validators:       validators,
+		proposer:         proposer,
+		totalVotingPower: totalVotingPower,
+	}
+}
+
+func ReadValidatorSet(r io.Reader, n *int64, err *error) *ValidatorSet {
+	size := ReadUInt64(r, n, err)
+	validators := []*Validator{}
+	for i := uint64(0); i < size; i++ {
+		validator := ReadValidator(r, n, err)
+		validators = append(validators, validator)
+	}
+	return NewValidatorSet(validators)
+}
+
+func (vset *ValidatorSet) WriteTo(w io.Writer) (n int64, err error) {
+	WriteUInt64(w, uint64(vset.validators.Size()), &n, &err)
+	vset.validators.Iterate(func(key_ interface{}, val_ interface{}) bool {
+		val := val_.(*Validator)
+		WriteBinary(w, val, &n, &err)
+		return false
+	})
+	return
+}
+
+func (vset *ValidatorSet) IncrementAccum() {
+	// Decrement from previous proposer
+	vset.proposer.Accum -= int64(vset.totalVotingPower)
+	var proposer *Validator
+	// Increment accum and find proposer
+	vset.validators.Iterate(func(key_ interface{}, val_ interface{}) bool {
+		val := val_.(*Validator)
+		val.Accum += int64(val.VotingPower)
+		proposer = proposer.CompareAccum(val)
+		return false
+	})
+	vset.proposer = proposer
+}
+
+func (vset *ValidatorSet) Copy() *ValidatorSet {
+	return &ValidatorSet{
+		validators:       vset.validators.Copy(),
+		proposer:         vset.proposer,
+		totalVotingPower: vset.totalVotingPower,
+	}
+}
+
+func (vset *ValidatorSet) GetById(id uint64) (index uint32, val *Validator) {
+	index_, val_ := vset.validators.Get(id)
+	index, val = uint32(index_), val_.(*Validator)
+	return
+}
+
+func (vset *ValidatorSet) GetByIndex(index uint32) (id uint64, val *Validator) {
+	id_, val_ := vset.validators.GetByIndex(uint64(index))
+	id, val = id_.(uint64), val_.(*Validator)
+	return
+}
+
+func (vset *ValidatorSet) Size() uint {
+	return uint(vset.validators.Size())
+}
+
+func (vset *ValidatorSet) TotalVotingPower() uint64 {
+	return vset.totalVotingPower
+}
+
+func (vset *ValidatorSet) Proposer() (proposer *Validator) {
+	return vset.proposer
+}
+
+func (vset *ValidatorSet) Hash() []byte {
+	return vset.validators.Hash()
+}
+
+func (vset *ValidatorSet) AddValidator(val *Validator) (added bool) {
+	if val.Accum != 0 {
+		panic("AddValidator only accepts validators with zero accumpower")
+	}
+	if vset.validators.Has(val.Id) {
+		return false
+	}
+	updated := vset.validators.Set(val.Id, val)
+	return !updated
+}
+
+func (vset *ValidatorSet) RemoveValidator(validatorId uint64) (removed bool) {
+	_, removed = vset.validators.Remove(validatorId)
+	return removed
+}